Head-up display (HUD) systems have been developed for aircraft in which the pilot can view the surroundings of the aircraft through a transparent screen on which is superimposed additional visually observable information in either alpha-numeric or symbolic form. Various types of display screens have been proposed, including holograms which transmit light from a projector toward the position of the observer's eyes. An example of such a system is shown in the U.S. Pat. No. to Freeman, 4,261,635.
Holographic optical element systems have also been developed which utilize a hologram to reflect or transmit light from a projector to a specific design eye location, such as within a cockpit of an airplane. The image is redirected only to an area at which the viewer may observe the image and is not directed to other areas, as exemplified in U.S. Pat. Nos. 4,798,426 and 5,037,166 to Malcolm and Neuperger, the disclosures of which are incorporated herein by reference. The display screen is a holographic optical element which behaves as a reflector or transmitter of high efficiency for a specific, narrow bandwidth of light and which may be translucent or optically clear to all other visible light.
A typical projector for projecting light onto a holographic optical element display screen for cockpit type displays is a cathode ray tube (CRT) with a phosphor coating having a spectral emission at or near the responsive wavelength of the display screen, and a relay lens assembly. Such systems are effective since the CRT projector is mounted relatively close to the display screen and the pilot is seated close to the display screen. However, the light projected from the CRT may not be sufficient in situations where the display screen is more distantly spaced from the projector or from an observer than is typical in airplane cockpit situations, or where it is desired to have a display screen that is substantially larger than those typically contained within a cockpit.